1. Field of the Invention
The present invention relates to a semiconductor optical waveguide device and a method for manufacturing the semiconductor optical waveguide device.
2. Description of the Related Art
U.S. Pat. No. 6,310,995 discloses an optical waveguide device including a spot size converter.
III-V group compound semiconductors have a refractive index of 3 or more for light in a near-infrared wavelength band used in optical communication or optical information processing. In an optical waveguide formed of a III-V group compound semiconductor having such a high refractive index, optical propagation in a fundamental transverse mode through the optical waveguide requires an optical waveguide width of 2 or less. The half-width in a light distribution of a transverse mode confined in the optical waveguide is much less than 2 μm.
The optical waveguide device such as a semiconductor optical modulator is optically coupled to a single-mode optical waveguide (for example, an optical fiber). The half-width (mode field diameter (MFD)) of light distribution of a transverse mode propagating in the optical fiber ranges from 3 to 4 μm, for example. Thus, the half-width of light distribution of a transverse mode propagating in a semiconductor optical waveguide is significantly different from the half-width of light distribution of a transverse mode propagating in an optical fiber. This difference causes an optical coupling loss between the semiconductor optical waveguide and the optical fiber. In order to reduce the optical coupling loss, a structure for increasing the half-width of light distribution (increasing the spot size) in a transverse mode, for example, a spot size converter (SSC) is provided in an input/output portion of a semiconductor waveguide device.
In the spot size converter of the optical waveguide device described in U.S. Pat. No. 6,310,995, a plurality of optical waveguide structures are included in the stacking direction of a stacked semiconductor layer. The spot size converter performs transition of guided light from one of the optical waveguide structures to another optical waveguide structure in the stacking direction.